1. Field of the Invention
The present invention relates to an image forming apparatus which mainly uses paper, film or cloth as a recording medium and records an image thereon.
2. Related Background Art
Heretofore, as recording methods for forming a monochromatic or color image, there have widely been used an electrophotography method and an ink jet method. Moreover, as a recording medium of these methods, an ink jet sheet is used in addition to ordinary paper. Among the ink jet sheets, there is used a recording paper having a special coating layer on a basic paper body. Such an ink jet sheet is carried to an image formation block normally by a duplo method using a recording medium feed roller and a friction pad or a retard method in which a recording medium is sandwiched by a normal rotation roller rotating in the recording medium conveyance direction and a reverse rotation roller rotating in the reverse direction, so that the paper is separated and supplied using a difference in the friction coefficients of these two rollers.
Moreover, these rollers have been made from a metal material and an elastic member made from urethane rubber.
However, conventionally, when the ink jet sheet is supplied to a predetermined image formation position by the aforementioned method, the ink jet sheet should be sandwiched by rollers to be carried to the image formation block. Moreover, when no image is to be recorded, the paper is usually sandwiched while waiting. Thus, when the paper is in contact with the rollers for a long time, the paper used as a recording medium may be contaminated. Especially in the case of the ink jet sheet, one side of the basic paper body has an absorption layer for absorbing an aqueous ink and this layer is easily contaminated by contact with the rubber roller. In an extreme case, this contamination changes a color of the image which has been recorded by the ink jet method.
Moreover, if sandwiched by the rollers for a long time, the roller causes a roller trace on the ink jet sheet and the roller trace may be seen after the image recording.
These problems extremely degrade the quality of the recorded image.
Moreover, the contamination results are different depending on the roller rubber type. In case of a rubber having a comparatively low hardness, the rubber contains much oil, which may come out from the roller onto the recording medium and contaminate the recording medium making a white spot. On the other hand, in case of a rubber having a comparatively high hardness, when the dedicated paper is in contact with the roller or even when a severalxe2x80x94millimeter space is present between them, the dedicated paper is often contaminated with a yellow color portion.
To solve these problems, the roller material has been modified as is disclosed in Japanese Patent Application No. 9-110213. However, the problems cannot be solved sufficiently when a medium other than paper such as a film and a cloth are used as the recording medium.
It is therefore an object of the present invention to provide an image forming apparatus that can prevent or reduce contamination of a recording medium caused by a recording medium conveying unit, regardless of the type of the recording medium; and a method for preventing contamination during the recording medium conveyance by the recording medium conveying unit.
Another object of the present invention is to provide a contamination preventing method applied during a recording medium conveyance in an image forming apparatus which holds a recording medium while conveying the recording medium and forming an image on the recording medium in such a manner that a contact portion of the recording medium is not held more than a predetermined time and the contact position of the recording medium is changed from time to time.